Apparatus for control of pellet size by relating work load to atomizer pressure



4, 1964 T. A. R. GULDMAN APPARATUS FOR CONTROL OF PELLET SIZE BYRELATING WORK LOAD T0 ATOMIZER PRESSURE Filed May 3, 1962 R cYcLE FINESHOT An? N o SLURRY DRIVER CURRENT METER POWER ATOMIZING REGULATOR KCONTROL AIR PELLETS INVENTQR moms A. .42. Gummy /'7 BY Ila/aw ATTORNEYUnited States Patent 3,142,862 APPARATUS FOR CONTROL OF PELLET SlZE BYRELATING WORK LOAD T0 ATOMIZER PRESSURE Thomas A. R. Guldman, anFrancisco, Calif., assignor to California Research Corporation, SanFrancisco, Calif, a corporation of Delaware Filed May 3, 1962, Ser. No.192,266 2 Claims. (Cl. 18-1) The present invention relates to animproved apparatus for preparing pellets of relatively infrangible orfracture resistant materials. More particularly, this invention relatesto a novel method of controlling the mean size distribution in theproduction of pellets of fracture resistant materials.

Granular particles of a small, defined size range may be prepared fromdispersions, i.e., slurries or solutions, in a continuous process. Theequipment is comprised of a rotating drum equipped with lifters on itswall, a hot air source and an atomizer and will be referred to as apelletizer. Hot air, a slurry or solution spray and pulverized recycledproducts, fines or nuclei, are introduced at one end and travelconcurrently down the drum. Recycled solid particles fall in a denseshower across the drum, being raised and evenly distributed by thelifters on its wall. The spray droplets impinge on these particles. Asthe particles carrying the slurry droplets fall through the hot airstream, the solvent, a vaporizable liquid, is evaporated, and theparticles reach the drum wall with a dry outer surface. The pitch of thedrum and flow of the hot gas act to move the particles toward the outletside of the drum.

An illustration of a comparable apparatus is found in US. Patent No.2,926,079, which is known in the industry as a spherodizer.

It is relatively impossible to obtain pellets all having the same sizeby this method. Nor is such a condition necessarily desirable, sincepellets of undesired size may be ground and returned to the drum asfines. Desirably then, a mean size distribution should be obtained whichoptimizes the desired size and provides sufficient improperly sizedpellets to be ground to fines.

During the operation of the above-described apparatus, the conditionsoften fluctuate. Particularly difficult is the problem of controllingthe solvent content and density of the dispersion. Variations in thecomposition of the dispersion will affect the droplet size which in turnafiects the mean distribution of the pellet size.

The drum of the apparatus is very long, often between 40 to 60 feet, andis capable of accommodating over 20,- 000 pounds of material. The timein which a particle moves through the drum will depend on the pitch ofthe drum, the particles size and other operating conditions, but willgenerally be in excess of minutes, usually in excess of 20 minutes.

The operator cannot see into the drum because of the denseness of theshower of fines. He must, therefore, wait until particles come out ofthe drum to be appraised of the improper functioning of the apparatus 20or more minutes before. He must then adjust the conditions, usually thepressure at the atomizing nozzle, until pellets of the properdistribution are obtained. This requires great sensitivity on the partof the operator in deciding the extent of the change. Moreover, duringthe period of time in which the operator was unaware of the undesirablesize distribution, and while adjustments are being made, tons ofmaterial may be formed which are commercially unusable and, when excessmoisture causes the material to agglomerate, may even result in theshutting down of the apparatus.

It has now been found that a proper pellet size dis- 3,142,862 PatentedAug. 4,, 1964 tribution may be maintained by making the pressure at theatomizing nozzle responsive to the work load in rotating the drum. Whenthe flow of solids through the drum is being maintained relativelyconstant, then any increase in the work load in rotating the drum at aconstant speed may be offset by an increase in pressure at the atomizingnozzle; a decrease in work load may be countered with a decrease inpressure.

The current or power fed to the driver motor varies directly as afunction of the Work load, which in turn is a function of the pelletsize distribution being produced. In any apparatus, a relationship maybe determined between the atomizing air pressure and the power orcurrent input to the driver motor, for maximizing the desired sizedistribution. By means of a meter, an operator may manually change theatomizing air pressure in relation to changes in the work load,determined by ammeter or wattmeter readings or, preferably, theatomizing air pressure may be varied automatically.

During operation of the pelletizer, it is important that the weight ofsolids introduced into the pelletizer be approximately equal to theweight of solids being removed. The rate at which the fine particles andslurry is introduced should be maintained in relation to the removal ofthe solid pellets from the pelletizer. Under optimum conditions, therewill be sufficient oversized and undersized pellets, which whenpulverized, will maintain a constant feed of fine particles. If there isan insuflicient amount of undesired size pellets, then pellets of thedesired size should be diverted to the pulverizer in order to maintainthe material balance.

In order to describe the novel method and apparatus further, referenceis now made to the accompanying drawings.

FIG. 1 represents an elevated view, partly in section, of an apparatusfor carrying out the process described herein.

Solid particles are fed from feeder 1 into the drum 2. A slurrycontaining fertilizer raw materials is fed through line 3 and sprayedinto the drum onto the fines through nozzle 4. The atomizing airpressure is introduced into the nozzle 4 through line 5, the pressurebeing regulated by valve 6. Hot air is blown into the drum at 7 andexits at 8.

The drum 2 is of a conventional rotary design, being belt (or gear)driven by means of motor 9 and belt (or gear) 10. The power fed to motor9 is metered by a driver current meter 11 which is connected to a powerregulator control 12. Variations in current activate the power regulatorcontrol 12 which actuates the valve 6 varying the atomizing air pressureat nozzle 4.

In operation, slurry is continuously sprayed into the drum 2 throughnozzle 4. The drum rotates at a suitable speed to shower the fines,which are raised by the lifters 13, through the spray of slurry and thehot gases are continuously blown through the drum 2 for distilling offthe solvent of the slurry, leaving the desired coated pellets. Shouldthe mean pellet size decrease or increase, the work necessary to turnthe drum and lift the nuclei and pellets will decrease or increaserespectively, resulting in a change in power to the motor 9. The changein power will actuate the valve control which will reduce or increasethe pressure producing larger or smaller spray droplets, respectively.

The work load in turning the drum is related to the power or currentinput to the driving motor. Various devices may be used to convert anelectric signal to pneumatic operation, for example, a Foxboro Dynalog.Because of the comparatively slow response of the change in powerrequirement to variations in the atomizing pressure, some adaptation ofa commercial instrument might be necessary.

The relationship between the power necessary to turn the drum and theatomizing air pressure may be determined in a variety of ways. A numberof settings may be made and the product size distribution determined ateach setting. The relationship between the current reading and atomizingair pressure may then be determined graphically and a plot made whichwould indicate the optimum settings for a desired distribution.

A less arduous procedure is to use the formula:

Air 1( Driver 2) where P is the atomizing air pressure, 1 is the drivermotor current and k and k are constants, which provides a convenientapproximation. The values of P and the corresponding values of 1 areobserved under two conditions under manual control. These values areinserted into the above equation, which may then be solved for k and kIf k; is small compared to I the simpler equation P =k I is obtained.

Pellets of various materials may be prepared, as long as the materialforms a relatively infrangible pellet. Because of the continued raisingand dropping of the pellets, it is necessary that the material does notpowder as a result of this mechanical action. Except for this onelimitation, pellets of many different materials may be prepared.Commercially this pelletizer is now used mainly for complex fertilizersbut other materials are well adapted to its use, e.g., detergents, highdensity insecticides, catalysts, etc.

Fertilizer pellets were prepared using an ammeter, indicated heretoforeas a driver current meter (11) in FIGURE 1, and manual control of theatomizing air pressure. The amperage was maintained in the range 5862amps. by varying the pressure in the range 65-70 p,s.i.g. Continuedproduction of the desired distribution was maintained for greater than24 hours, whereas prior to this, production had been frequently reducedafter periods of 6 hours, due to gross deviations from the desired sizedistribution. This meant that 300 tons of the desired pellets wereproduced without interruption, Where previously only about 75 tons wereproduced before interruption. The deviations also overload the recyclesystem, as well as reduce the yield of the desired size pellets.

As will be evident to those skilled in the art, various modifications onthis process can be made or followed, in the light of the foregoingdisclosure and discussion, without departing from the spirit or scope ofthe disclosure or from the scope of the following claims.

I claim:

1. In an apparatus for preparing pellets of a fracture resistantmaterial which comprises a rotating drum equipped with lifters on itswall, means for rotating said drum at a constant rate, means forintroducing fines into the drum, an atomizer for spraying droplets of adispersion of the fracture resistant material onto the fines as theyfall from the lifters across the drum, pressure means operably connectedto said atomizer for controlling the size of the droplets, means formoving said coated fines through said drum, and means for drying thecoated fines with hot gas traveling concurrently with the coated finesthrough said drum, the improvement which comprises means for varyingsaid atomizer pressure responsive in direct relation to the work load ofsaid rotating means.

2. In an apparatus for preparing pellets of a fracture resistantmaterial which comprises a rotating drum equipped with lifters on itswall, means for rotating said drum at a constant rate, means forintroducing fines into the drum, an atomizer for spraying droplets of adispersion of the fracture resistant material onto the fines as theyfall from the lifters across the drum, pressure means operably connectedto said atomizer for controlling the size of the droplets, means formoving said coated fines through said drum, and means for drying thecoated fines with hot gas traveling concurrently with the coated finesthrough said drum, the improvement which comprises maintaining thepellets in a relatively constant size dis tribution with means formetering the work load in rotating said drum and means for varying theatomizing pressure in direct relation to variation in the work load,said atomizer pressure varying means being operably connected to saidmetering means.

References Cited in the file of this patent UNITED STATES PATENTS2,800,399 King July 23, 1957 2,926,079 Smith Feb. 23, 1960 2,980,526Wilson Apr. 18, 1961 3,034,421 Pence May 15, 1962

1. IN AN APPARATUS FOR PREPARING PELLETS OF A FRACTURE RESISTANTMATERIAL WHICH COMPRISES A ROTATING DRUM EQUIPPED WITH LIFTERS ON ITSWALL, MEANS FOR ROTATING SAID DRUM AT A CONSTANT RATE, MEANS FORINTRODUCING FINES INTO THE DRUM, AN ATOMIZER FOR SPRAYING DROPLETS OF ADISPERSION OF THE FRACTURE RESISTANT MATERIAL ONTO THE FINES AS THEYFALL FROM THE LIFTERS ACROSS THE DRUM, PRESSURE MEANS OPERABLY CONNECTEDTO SAID ATOMIZER FOR CONTROLLING THE SIZE OF THE DROPLETS,MEANS FORMOVING SAID COATED FINES THROUGH SAID DRUM, AND MEANS FOR DRYING THECOATED FINES WITH HOT GAS TRAVELING CONCURRENTLY WITH THE COATED FINESTHROUGH SAID DRUM, THE IMPROVEMENT WHICH COMPRISES MEANS FOR VARYINGSAID ATOMIZER PRESSURE RESPONSIVE IN DIRECT RELATION TO THE WORK LOAD OFSAID ROTATING MEANS.